Fiber bed mist eliminators have found wide industrial application in the removal of aerosols from gas streams. Some of the more frequent applications include removal of acid mists, such as sulfuric acid mists, in acid manufacturing, removal of plasticizer mists in the manufacture of polyvinyl chloride floor or wall coverings and removal of water soluble solid aerosols from the emissions of ammonium nitrate prill towers. In these various applications, fiber bed mist eliminators may achieve separation efficiencies of 99% or greater.
It is generally known that fibers made of various materials may be utilized to construct fiber beds for fiber bed mist eliminators. The fiber bed is designed to collect fine liquid mist and soluble solid particles entrained in a moving gas stream and drain them through the structure of the bed. Typically, beds of collecting fibers are associated with metal wire screens or similar external support structures. The combination of a bed of collecting fibers and external support structure is known as a fiber bed element. As used hereinafter, fiber bed refers to that portion of the fiber bed element apart from any such support structure. Fiber beds may be formed by packing bulk fiber between two opposing support screens (bulk-packed beds) or winding a roving made of fibers around a cylindrical support screen (wound beds). Although not limited to such a configuration, fiber bed elements are most often configured in the form of a substantially vertical cylinder. Cylindrical fiber bed elements permit a high effective fiber bed surface area in a minimum of space. Flat fiber bed elements on the other hand, find particular application in high gas velocity applications.
In operation, a substantially horizontal stream of gas containing a liquid or soluble solid aerosol is made to penetrate and pass through the fiber bed of the fiber bed element. The fibers in the fiber bed capture the aerosol in the gas by the mechanisms of impaction, interception, and Brownian diffusion. The captured aerosol coalesces on the fibers to form droplets of liquid in the fiber bed. The moving gas urges the droplets to move toward the downstream face of the fiber bed where the captured liquid exits the fiber bed and drains downward under the force of gravity.
The fibers which comprise the fiber bed may be made from a variety of materials. Materials utilized to make bed fiber include, for example, metals such as stainless steel, titanium, etc., fibers of polymeric materials such as polyesters, polyvinylchloride, polyethylene terphthalate, nylons, polyethylene, polypropylene etc., and glass. In applications where corrosive conditions and/or high temperatures are encountered, chemical grade glass fibers have found particularly widespread use in fiber beds of fiber bed mist eliminators.
Fibers ranging in diameter from 5 .mu.m or less to more than 200 .mu.m, as well as combinations of fibers of varying diameters, have been utilized in fiber beds. The bulk density of prior art fiber beds ranges from about 5 lb/ft.sup.3 to greater than 20 lb/ft.sup.3, while fiber bed thickness ranges from about 0.5 to about 4 inches or more, depending upon the desired separation efficiency.
In order for a fiber bed to function effectively, the bed must be mechanically stable. A mechanically stable fiber bed is one which will retain its structural integrity without substantial shifting of the fibers relative to adjacent fibers when exposed to the forces exerted by the gas being treated and the captured and draining liquid during aerosol collection. If mechanical stability is not maintained the performance characteristics of the bed will be unfavorably altered. In a fiber bed lacking mechanical stability, the moving gas stream forces the fibers to shift substantially, causing the liquid ladened fibers in some portions of the fiber bed to mat or felt while increasing the void space between adjacent fibers in other portions of the fiber bed. Matted portions of the fiber bed are more resistant to both the flow of gas and the drainage of captured liquid. Captured liquid which is unable to drain is often reentrained by the moving gas stream, resulting in reduced separation efficiency. Also, the pressure drop across matted portions of an unstable fiber bed is increased. On the other hand, in those portions of a mechanically unstable fiber bed where fiber shifting has increased the average void space between adjacent fibers, macroscopic pores or channels form which reduce separation efficiency by allowing the aerosol containing gas to pass through the fiber bed without sufficient contact with the collecting fibers.
In bulk-packed and wound fiber bed elements, mechanical stability is largely dependent upon the bulk density of the fiber bed. In these conventionally constructed fiber bed elements, a fiber bed bulk density within the range disclosed by the prior art typically provides sufficient contact between adjacent fibers to prevent substantial movement of the fibers when exposed to the forces exerted by the moving gas stream. In bulk-packed fiber beds, density of the fiber bed and resistance to fiber movement is maintained by the reactive compressive force applied against the packed fibers by the two opposing support screens. In wound fiber beds, density of the fiber bed necessary to provide mechanical stability is a result of several factors including the tension in the roving as it is wound around the cylindrical support screen and compression of the fiber bed by a wire screen or similar structure which may be wound on the cylinder adjacent to the exterior surface of the fiber roving.
However, bulk density cannot be increased indiscriminately to achieve mechanical stability. If the bulk density of a fiber bed is increased too much, the bed will be prone to flooding. An effective fiber bed is a relatively open structure that allows free gas flow and liquid drainage even when the collecting fibers are coated with collected liquid. There must be sufficient void space between adjacent fibers in the bed so that collected liquid is not able to bridge the space between adjacent fibers to such an extent that the adherence of collected liquid to the surface of fibers prevents the liquid from draining.
A measure of the open space in a fiber bed is void fraction which is defined by the bulk density of the fiber bed and the average density of the fiber material according to the following formula: ##EQU1## Fiber beds typically have a void fraction of greater than about 0.89.
It is generally known that the thickness of a fiber bed can be decreased without a loss in separation efficiency by decreasing the average fiber diameter of the fiber material comprising the fiber bed. However, for bulk-packed and wound fiber beds comprised of fibers having an average diameter of less than about 5 .mu.m, when the bulk density is high enough to ensure mechanical stability, the resulting void fraction is so low that the bed tends to flood under typical operating conditions. A flooded bed is a fiber bed in which captured liquid substantially fills the void spaces between adjacent fibers in the fiber bed. A flooded fiber bed is much like the matted portions of an unstable fiber bed. The captured liquid in a flooded fiber bed can not properly drain and instead may be reentrained in the moving gas stream at the downstream face of the fiber bed. Furthermore, the pressure drop across a fiber bed element is increased when the fiber bed is flooded. If a pressure differential across the fiber bed sufficient to overcome the force of adhesion and dislodge the collected droplets from the fibers is employed, the collected liquid is blown from the downstream face of the fiber bed where it is reentrained by the gas stream resulting in low separation efficiency and increased operating cost.
In order to prevent a fiber bed comprised of small average diameter fibers from flooding, the specific fiber surface area, expressed as the area of fiber per unit volume of the fiber bed, may be decreased by decreasing the bulk density of the bed (i.e., increasing the void fraction). However, if the bulk density of a bulk-packed or wound fiber bed comprised of fibers having an average diameter less than about 5 .mu.m is reduced to a value sufficient to avoid flooding, such fiber beds lack the mechanical stability necessary to withstand the forces exerted by the moving gas stream. As a result, the moving gas stream causes the fibers to shift substantially resulting in the fiber bed matting and/or channeling as previously described. Therefore, in practice, conventionally constructed high efficiency fiber bed elements comprise fiber beds 2 to 4 inches thick constructed of fibers having an average fiber diameter between 5 and 15 .mu.m and having a bulk density between 5 and 15 lb/ft.sup.3.
In contrast to fiber beds used in mist eliminators, some other types of gas filters, such as baghouse, clean room and breathing filters, may successfully utilize glass fibers with average diameters below 5 .mu.m and may even include fibers having a diameter less than 1 .mu.m. However, these types of gas filters are distinguished from fiber bed mist eliminators in that they are typically designed to utilize pore and surface filtration in collecting solid particulates or only relatively small amounts of liquid aerosols. If used to collect liquid aerosols, they easily flood at the liquid loading rates typically encountered in fiber bed mist eliminators. By comparison, fiber bed mist eliminators are designed to allow comparatively large quantities of liquid entrained in a moving gas stream to penetrate the fiber bed where the liquid is captured and continuously drained. Therefore, a solution to the problems associated with utilizing small diameter fibers in fiber beds of fiber bed mist eliminators is neither shown nor suggested by the prior art concerned with such other gas filters.
Reentrainment of collected liquid by the moving gas stream at the downstream face of a fiber bed is often a problem in fiber bed mist eliminator applications, especially in operations characterized by high liquid loading rates or high gas stream velocities. Satisfactory solutions to this problem have included combining a layer of primary filtration fibers and a layer of drainage fibers to form a fiber bed. The drainage layer is downstream of the primary filtration layer and usually comprised of fibers with a larger average diameter than those fibers comprising the primary filtration layer.
Despite their success, fiber bed elements of the prior art have several disadvantages resulting from shortcomings in the fiber bed. Conventionally constructed fiber bed elements include substantial external support structures of metal, fiberglass, polypropylene, etc. necessary to maintain a fiber bed bulk density sufficient to provide mechanical stability. A fiber bed element containing a bulk-packed fiber bed must include two opposing support screens substantial enough to retain the fibers in compression and thereby maintain fiber bed density. Wound fiber bed elements must contain a cylindrical support structure able to withstand the forces exerted by the roving during manufacture of the fiber bed element. Furthermore, because conventional fiber beds do not have form absent the external support structure of the fiber bed element, such support structure is needed in both bulk-packed and wound fiber bed elements to hold the fiber bed in the desired configuration. The need for substantial external support structure is a cost disadvantage which in many instances is worsened by the fact that the structure must be made from expensive corrosion resistant materials such as stainless steel alloys and high performance fiberglass resins.
Another disadvantage associated with bulk-packed and wound fiber bed elements is the need to remove the elements from the mist eliminator and return them to the manufacturer when the fiber bed needs replacement. Aside from the inconvenience of transporting the heavy support structure, an interchangeable spare fiber bed element must be available for the fiber bed mist eliminator to be returned to service and avoid extensive down time. Field replacement of the fiber bed is hampered by difficulties in assembling the fiber bed. Considerable skill and time are required to properly pack new bulk fiber between opposing support screens so that inhomogeneities in the fiber bed are avoided, while winding fiber onto a cylindrical support structure requires extensive machinery. Precast fiber bed sections have been used by some manufacturers but these have suffered from leakage at joints or settling.
A third disadvantage is that because fibers having an average fiber diameter of less than about 5 .mu.m can not be used effectively in constructing a conventional fiber bed, fiber bed thickness in applications requiring high separation efficiency can not be reduced. If thinner high efficiency fiber beds were possible, fiber beds could be configured into shapes that maximize the fiber bed surface area in a given volume available for a fiber bed mist eliminator. This would be analogous to the dry filter art where thin filter papers and felts allow high surface area filter forms via pleating. Thinner, high efficiency fiber beds having increased fiber bed surface area would allow the operating cost of fiber bed mist eliminators to be decreased by decreasing the pressure drop across the fiber bed.